Electrode impedance spectroscopy

ABSTRACT

Aspects of the present invention are generally directed to impedance spectroscopy in an active implantable medical device (AIMD) comprising a component with one or more electrodes. In an embodiment, the AIMD applies a signal at a plurality of frequencies using one or more of the electrodes. Measurements are then obtained for the applied signal to determine impedance(s) at the applied frequencies of the tissue in which the electrodes are located.

BACKGROUND

Field of the Invention

The present invention relates generally to implantable stimulatorsystems, and more particularly, to impedance measurements in animplantable stimulator system.

Related Art

Implantable medical devices have provided benefits to recipients overrecent decades. Implantable medical devices are devices having one ormore components or elements that are at least partially implantable in arecipient. One type of implantable medical device is an activeimplantable medical device (AIMDs), which are medical devices having oneor more implantable components that rely for their functioning upon asource of power other than the human body or gravity, such as anelectrical energy source. Exemplary AIMDs include devices configured toprovide one or more of stimulation and sensing, such as implantablestimulator systems and implantable sensor systems.

Implantable stimulator systems provide stimulation to a recipient of thedevice. Exemplary implantable stimulator systems include, but are notlimited, to cochlear implants, auditory brain stem implants, cardiacpacemakers, neurostimulators, functional electrical stimulation (FES)systems, etc.

Cochlear implants include an electrode assembly implanted in the cochleaand are used to treat sensorineural hearing loss. Electrical stimulationsignals are delivered directly to the auditory nerve via the electrodeassembly, thereby inducing a hearing sensation in the implant recipient.

An Auditory Brain Stem Implants (ABI) is another type of surgicallyimplanted electronic device that provides a sense of sound to arecipient suffering from sensorineural hearing loss. ABIs are typicallyused in recipients suffering from sensorineural hearing loss that, dueto damage to the recipient's cochlea or auditory nerve, are unable touse a cochlear implant.

A cardiac pacemaker is a medical device that uses electrical impulses,delivered by electrodes contacting the heart muscles, to regulate thebeating of a heart. The primary purpose of a pacemaker is to maintain anadequate heart rate.

A neurostimulator, also sometimes referred to as an implanted pulsegenerator (IPG) is a battery powered device designed to deliverelectrical stimulation to the brain. Neurostimulators are sometimes usedfor deep brain stimulation and vagus nerve stimulation to treatneurological disorders.

FES uses electrical currents to activate nerves innervating extremitiesaffected by paralysis resulting from, for example, spinal cord injury,head injury, stroke, or other neurological disorders.

Other types of implantable stimulator systems include systems configuredto provide electrical muscle stimulation (EMS), also known asneoromuscular stimulation (NMES) or electromyostimulation, whichinvolves the application of electric impulses to elicit musclecontraction.

Exemplary implantable sensor systems include, but are not limited to,sensor systems configured to monitor cardiac, nerve and muscularactivity.

SUMMARY

In one aspect of the invention, there is provided a method of operatingan active implantable medical device (AIMD) comprising an electrode, themethod comprising: applying a measurement signal at a plurality offrequencies to a recipient of the AIMD using the electrode; andperforming, using the AIMD, a measurement, responsive to the measurementsignal and indicative of an impedance of the electrode and tissue of therecipient, at each of the plurality of frequencies.

In another aspect, there is provided an active implantable medicaldevice comprising: a component comprising an electrode; a signalgenerator configured to apply a signal using the electrode at aplurality of frequencies; and a control circuit configured to perform ameasurement, responsive to the signal, indicative of an impedance of theelectrode and tissue of the recipient for each of the plurality offrequencies.

In yet another aspect, there is provided a system for performingspectroscopy comprising: an active implantable medical device (AIMD)comprising: an electrode; means for applying a measurement signal to arecipient using the electrode at a plurality of frequencies; and meansfor performing, using the AIMD, a measurement, responsive to themeasurement signal and indicative of an impedance of the electrode andtissue of the recipient for each of the plurality of frequencies.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the present invention are described hereinwith reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a cochlear implant system in whichembodiments of the present invention may be implemented;

FIG. 2 is a functional block diagram of the cochlear implant system ofFIG. 1, in accordance with an embodiment of the present invention;

FIG. 3A is a simplified diagram of an exemplary stimulating leadassembly, in accordance with an embodiment of the present invention;

FIG. 3B illustrates the stimulating lead assembly of FIG. 3A inserted ina cochlea;

FIG. 4 provides a simplified diagram of exemplary circuitry forimpedance spectroscopy in a cochlear implant, in accordance with anembodiment of the present invention;

FIG. 5 is a flow chart of an exemplary 500 for obtaining impedancemeasurements, in accordance with an embodiment of the present invention;

FIG. 6A illustrates a platinum electrode surrounded by a cellular medium(i.e. tissue);

FIG. 6B illustrates an equivalent circuit model 600 of the systemillustrated in FIG. 6A;

FIG. 7A provides a diagram for illustrating how the uneven surface of anelectrode at the microscopic level can be modeled as a CPE;

FIG. 7B provides a diagram of a circuit model for a CPE in accordancewith FIG. 7A;

FIG. 8 is an exemplary plot for an electrode surrounded by a cellularmedium, such as shown in FIG. 6A; and

FIG. 9 illustrates a square wave current pulse and a resulting measuredvoltage 904, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention are generally directed to impedancespectroscopy in an active implantable medical device (AIMD) comprising acomponent with one or more electrodes. In an embodiment, the AIMDapplies a signal at a plurality of frequencies using one or more of theelectrodes. Measurements are then taken for the applied signal. Thesemeasurements are used to measure impedance(s) at the applied frequenciesof the tissue in which the electrodes are located. The measuredimpedances are then analyzed to determine, for example, the proximity ofthe electrodes to one or more biological structures of the recipient.This information may be used by a surgeon during implantation of thecomponent of the AIMD in the recipient or by a clinician at any time todiagnose potential problems with or to confirm correct operation of oneor more electrodes of the AIMD or the tissue surrounding them.

Embodiments of the present invention are described herein primarily inconnection with one type of Active Implantable Medical Device (AIMD),namely a cochlear implant system (commonly referred to as cochlearprosthetic devices, cochlear prostheses, cochlear implants, cochleardevices, and the like; simply “cochlea implant systems” herein.)Cochlear implant systems generally refer to hearing prostheses thatdeliver electrical stimulation to the cochlea of a recipient. As usedherein, cochlear implant systems also include hearing prostheses thatdeliver electrical stimulation in combination with other types ofstimulation, such as acoustic or mechanical stimulation. It would beappreciated that embodiments of the present invention may be implementedin other types of AIMDs.

FIG. 1 is perspective view of a cochlear implant system, referred to ascochlear implant system 100 implanted in a recipient. FIG. 2 is afunctional block diagram of cochlear implant system 100. The recipienthas an outer ear 101, a middle ear 105 and an inner ear 107. Componentsof outer ear 101, middle ear 105 and inner ear 107 are described below,followed by a description of cochlear implant system 100.

In a fully functional ear, outer ear 101 comprises an auricle 110 and anear canal 102. An acoustic pressure or sound wave 103 is collected byauricle 110 and channeled into and through ear canal 102. Disposedacross the distal end of ear cannel 102 is a tympanic membrane 104 whichvibrates in response to sound wave 103. This vibration is coupled tooval window or fenestra ovalis 112 through three bones of middle ear105, collectively referred to as the ossicles 106 and comprising themalleus 108, the incus 109 and the stapes 111. Bones 108, 109 and 111 ofmiddle ear 105 serve to filter and amplify sound wave 103, causing ovalwindow 112 to articulate, or vibrate in response to vibration oftympanic membrane 104. This vibration sets up waves of fluid motion ofthe perilymph within cochlea 140. Such fluid motion, in turn, activatestiny hair cells (not shown) inside of cochlea 140. Activation of thehair cells causes appropriate nerve impulses to be generated andtransferred through the spiral ganglion cells (not shown) and auditorynerve 114 to the brain (also not shown) where they are perceived assound.

Cochlear implant system 100 comprises an external component 142 which isdirectly or indirectly attached to the body of the recipient, and aninternal component 144 which is temporarily or permanently implanted inthe recipient. External component 142 typically comprises one or moresound input elements, such as microphone 124 for detecting sound, asound processor 126, a power circuit (not shown), and an externaltransmitter unit 128. External transmitter unit 128 comprises anexternal coil 130 and, preferably, a magnet (not shown) secured directlyor indirectly to external coil 130. Sound processor 126 processes theoutput of microphone 124 that is positioned, in the depicted embodiment,by auricle 110 of the recipient. Sound processor 126 generates encodedsignals, sometimes referred to herein as encoded data signals, which areprovided to external transmitter unit 128 via a cable (not shown). Soundprocessor 126 may further comprise a data input interface (not shown)that may be used to connect sound processor 126 to a data source, suchas a personal computer or musical player (e.g., an MP3 player).

Internal component 144 comprises an internal receiver unit 132, astimulator unit 120, and a stimulating lead assembly 118. Internalreceiver unit 132 comprises an internal coil 136, and preferably, amagnet (also not shown) fixed relative to the internal coil. Internalreceiver unit 132 and stimulator unit 120 are hermetically sealed withina biocompatible housing, sometimes collectively referred to as astimulator/receiver unit. The internal coil receives power andstimulation data from external coil 130. Stimulating lead assembly 118has a proximal end connected to stimulator unit 120, and a distal endimplanted in cochlea 140. Stimulating lead assembly 118 extends fromstimulator unit 120 to cochlea 140 through mastoid bone 119. In someembodiments stimulating lead assembly 118 may be implanted at least inbasal region 116, and sometimes further. For example, stimulating leadassembly 118 may extend towards apical end of cochlea 140, referred toas cochlea apex 134. In certain circumstances, stimulating lead assembly118 may be inserted into cochlea 140 via a cochleostomy 122. In othercircumstances, a cochleostomy may be formed through round window 121,oval window 112, promontory 123 or through an apical turn 147 of cochlea140.

Stimulating lead assembly 118 comprises a longitudinally aligned anddistally extending array 146 of electrodes 148 (also referred to aselectrode contacts), sometimes referred to as array of electrodes 146 orarray of electrode contacts 146 herein. Although array of electrodes 146may be disposed on stimulating lead assembly 118, in most practicalapplications, array of electrodes 146 is integrated into stimulatinglead assembly 118. As such, array of electrodes 146 is referred toherein as being disposed in stimulating lead assembly 118. Stimulatorunit 120 generates stimulation signals which are applied by electrodes148 to cochlea 140, thereby stimulating auditory nerve 114. Because, incochlear implant system 100, stimulating lead assembly 118 providesstimulation, stimulating lead assembly 118 is sometimes referred to as astimulating lead assembly. Stimulator unit 120 may further be connectedto an extra-cochlear electrode (not shown) located external to therecipient's cochlea 140.

In cochlear implant system 100, external coil 130 transmits electricalsignals (that is, power and stimulation data) to internal coil 136 via aradio frequency (RF) link. Internal coil 136 is typically a wire antennacoil comprised of multiple turns of electrically insulated single-strandor multi-strand platinum or gold wire. The electrical insulation ofinternal coil 136 is provided by a flexible silicone molding (notshown). In use, implantable receiver unit 132 may be positioned in arecess of the temporal bone adjacent auricle 110 of the recipient.

FIG. 3A is a simplified diagram of an exemplary stimulating leadassembly 318, in accordance with an embodiment of the present invention.FIG. 3B illustrates stimulating lead assembly 318 inserted in cochlea140. As illustrated, stimulating assembly 318 is configured to adopt acurved configuration during and or after implantation into therecipient's cochlea 140. To achieve this, in certain embodiments,stimulating assembly 318 is pre-curved to the same general curvature ofa recipient's cochlea 140. In such embodiments, stimulating assembly 318is sometimes referred to as perimodiolar stimulating assembly wherestimulating assembly 318 adopts its curved configuration in cochlea 140.When implanted, the surface of stimulating lead assembly 318 that facesthe interior of cochlea 140 is referred to herein as the medial surfaceof stimulating lead assembly 318. As illustrated, electrodes 348 arelocated on the medial side of stimulating lead assembly 318. Further, asshown, when implanted the tip 350 of stimulating lead assembly 318 islocated near the cochlear apex 134.

Although FIGS. 3A-3B illustrate a perimodiolar stimulating assembly. Inother embodiments, stimulating assembly may be a non-perimodiolarstimulating assembly which does not adopt a curved configuration. Forexample, stimulating assembly 318 may comprise a straight stimulatingassembly or a mid-scala assembly which assumes a mid-scala positionduring or following implantation. In further embodiments, cochlearimplant could include a stimulating assembly implantable into a naturalcrevice in the cochlea that allows for the hydrodynamic nature of thecochlea to be maintained, or an assembly positioned adjacent to thecochlea.

The state of the tissue and anatomical structures surrounding theelectrodes of a stimulating lead assembly is typically an importantfactor with regard to the effectiveness of stimulation delivered to therecipient. Present stimulator systems measure the impedance of eachelectrode, which provides a useful measure that assists clinicians indiagnosing faults, assessing the position of the electrode anddetermining anatomical anomalies in the cochlea. This impedance iscurrently reported as a single number and thus provides limitedinformation about the state of the tissue surrounding the electrode. Forexample, this impedance is typically determined at what is essentially asingle frequency and used for the limited purpose of fault detection incochlear implants.

As will be discussed further below, an embodiment of the presentinvention, measures the impedance of an electrode and its surroundingtissue over a range of frequencies (referred to herein as impedancespectroscopy) to obtain a more detailed picture of the state of theelectrode and its surrounding tissue (including the surroundinganatomical structures).

These impedance measurements may be used in a plurality of applications.For example, these impedance measurements may be used to determine theproximity of an electrode to different physical structures (e.g., themodiolus, the lateral wall, etc.) within the cochlea since differentphysical structures may present different spectroscopic signatures. Thisinformation may be used during surgical implantation, for example, toprovide the surgeon with information regarding the position of thestimulating lead assembly in the recipient's cochlea. For example, thisinformation may be used to determine the instantaneous insertion depthof the stimulating lead assembly 318. Further, post surgery, thisinformation (e.g., the proximity of the electrodes to the modiolus) maybe useful in predicting performance of the cochlear implant.

Impedance spectroscopy in accordance with embodiments of the presentinvention may also be useful in detecting issues with the tissuesurrounding the stimulating lead assembly, such as assessing the extentof scar tissue around the electrodes or the presence of an infectionnear the electrode. Additionally, during surgical implantation impedancespectroscopy in accordance with embodiments may be useful in detectingtip fold-over (i.e., when tip 350 folds back on stimulating leadassembly 318 during the insertion process). Impedance spectroscopy inaccordance with embodiments may also be useful in detecting otherstimulating lead assembly faults (e.g., faults more complex than simpleopen circuits or short circuits) as will be discussed in more detailbelow.

As noted above, an embodiment measures the impedance of an electrode andits surrounding tissue over a range of frequencies to obtain a moredetailed picture of the state of the electrode and its surroundingtissue (including the surrounding anatomical structures). In anembodiment, these impedances are measured by measuring the impedancebetween a pair of electrodes (e.g., an electrode of the stimulating leadassembly and an extra-cochlear electrode) of a stimulating lead assemblyover the frequency range. The measurements may be used to generateimpedance spectroscopy plots. The characteristics of the tissue close tothe electrodes can have a significant impact on the measured impedances,and thus the shape of generated impedance spectroscopy plots. Forexample, the close proximity of the modiolar wall to the electrode mayhave a profound effect on the shape of the plot compared to the plot'sshape when the electrode is surrounded by perilymph, the fluid whichoccupies the space inside the cochlea 140. Thus, a plot of measurementswhen the electrode is near the modiolus will look quite different from aplot of measurements taken when the electrode is distant from it andsurrounded by perilymph. As will be discussed further below, anembodiment of the present invention uses impedance spectroscopy toobtain an impedance spectroscopy plot and then compares the obtainedplot with different characteristic plot shapes to obtain informationregarding the stimulating lead assembly, such as, for example, theproximity of the electrode(s) to various cochlear structures,characteristics of the tissue surrounding the electrode(s), issues withthe stimulating lead assembly (e.g., tip fold-over, faults, etc.).

FIG. 4 provides a simplified diagram of exemplary circuitry forimpedance spectroscopy in a cochlear implant, in accordance with anembodiment of the present invention. For ease of explanation only thecomponents discussed below are illustrated in FIG. 4. As illustrated,stimulator unit 420 is connected to internal receiver unit 432,electrode 448 and extra-cochlear electrode 450. Stimulator unit 420,internal receiver unit 432, electrode 448, extra-cochlear electrode 450may be, for example, components such as the correspondingly namedstimulator unit 120, internal receiver unit 132, electrode 148 andextra-cochlear electrode (not shown) discussed above with reference toFIGS. 1-2.

As illustrated, stimulator unit 420, includes a control circuit 402, asignal generator 404, a resistor 406, and a voltage measurement circuit408. Control circuit 402 may be a circuit (e.g., an Application SpecificIntegrated Circuit (ASIC)) configured for exercising control over thestimulator unit 420. For example, control circuit 402 may be configuredfor receiving, from the internal receiver unit 432, the encoded datasignals regarding the sound and generating the stimulating signals forapplying stimulation via electrodes 448 and 450.

Signal generator 404 generates a voltage for application via thestimulating lead assembly. For ease of explanation, signal generator 404is a separate voltage generator distinct from the electronics used forapplying stimulation to cause a hearing percept by the recipient. Itshould be noted that FIG. 4 provides but one simple example of anembodiment for performing impedance spectroscopy in accordance with thepresent invention and that in other embodiments other types of systemsand components may be used. For example, as will be discussed furtherbelow, another embodiment uses the existing current generator present intypical cochlear implants for generating the signal in place of thevoltage generator used in the embodiment of FIG. 4.

As illustrated, a voltage measurement circuit 408 is connected toopposite ends of resistor 406. Resistor 406 may be a standard resistor,such as, for example, a 100 ohm resistor. Voltage measurement circuit408 may include any type of circuitry configured to output a signalindicative of the voltage across resistor 406. For example, in anembodiment, voltage measurement circuit 408 may comprise a differentialamplifier that takes as inputs the signals on opposite sides of resistor406 and then amplifies the difference in the voltage between the twosides. Voltage measurement circuit 408 provides the measured voltage tocontrol circuit 402. Further, in embodiments, voltage measurementcircuit 408 may comprise an analog to digital converter (ADC) thatdigitizes the measured voltage before providing the measured voltage tothe control circuit 402.

FIG. 5 provides a flow chart of an exemplary method 500 for obtainingimpedance measurements, in accordance with an embodiment of the presentinvention. Control circuit 402, at block 502, initiates the process formeasuring impedances. In an embodiment, the process is performed duringsurgical implantation to obtain information regarding the location ofthe stimulating lead assembly in the recipient's cochlea during thesurgical implantation procedure. In such an embodiment, the electrode(s)of the stimulating lead assembly may be connected to an external deviceconfigured to aid the surgeon during the implantation process. In theillustrated embodiment of FIG. 4, the external device may be, forexample, connected to the sound processor 126 (FIG. 1), connected to theinternal receiver unit 132, or connected to the stimulator unit 120. Insuch examples, the connection between the external device and thecomponent of the cochlear implant may be via a wired or wirelessconnection. In such embodiments, the process may be initiated by acommand (e.g., initiated by the surgeon) being transmitted from theexternal device to the control circuit 402 to initiate the process. Afurther description of exemplary external devices is provided below.

In another embodiment, the external device may be directly connected tothe leads connected to the electrode(s), such as, for example, inembodiments in which the stimulating lead assembly is implanted prior toconnecting the stimulating lead assembly to the stimulator unit. In suchembodiments, the control circuit 402, signal generator 404, resistor406, and voltage measurement circuit 408 may be implemented in theexternal device.

Or, for example, in embodiments in which the process is performed aftersurgical implantation of the stimulating lead assembly, a clinician mayconnect to the sound processing unit 126 and direct sound processingunit 126 to send a command to the stimulator unit 420 to initiate theprocess. In another embodiment, control circuit 402 may, for example,determine to initiate the process based on an amount of time that haselapsed since the last measurement (e.g., the control circuit 402performs measurements once a day, week, month, etc.). Or, for example,control circuit 402 may monitor performance of the stimulator unit 420and initiate the process if a particular event occurs (e.g., a fault isdetected).

In the presently discussed embodiment, at block 504, control circuit 402selects the first pair of electrodes 448 and 450 for which impedancemeasurements are to be taken. The selected pair of electrodes 448 and450 may be a pair of electrodes that is used for application ofstimulation via monopolar stimulation, where current flows from anelectrode 448 of the stimulating lead assembly to an extra-cochlearelectrode 450. Or, for example, (e.g., in systems that use bipolarstimulation) both electrodes of the pair may be electrodes of thestimulating lead assembly.

In an embodiment, the stimulating lead assembly comprises 22 electrodes,where each electrode is paired with the extra-cochlear electrode toprovide 22 separate stimulation channels. In one such embodiment,control circuit 402 performs impedance measurements for each of the 22separate stimulation channels using the electrode pairs corresponding toeach stimulation channel. Control circuit 402 may select, for example, apair of electrodes corresponding to one of these stimulation channels asthe first selected pair of electrodes and then in subsequent passes,control circuit 402 may select the electrode pairs for the otherstimulation channels.

For each selected electrode pair, control circuit 402 takes impedancemeasurements for a plurality of frequencies. In an embodiment, thesefrequencies are spaced across the operational frequency range of thedevice. However, in other embodiments, the frequencies may includefrequencies outside the normal operation range of the device.

The frequency of the applied sinusoidal voltage may be swept from low tohigh or high to low in a number of steps and measurements of thesinusoidal current amplitude and phase taken at each frequency step. Forexample, in a system where the operation frequency range is between 50and 20 kHz (i.e., a frequency sweep ranging from 50-20 kHz), controlcircuit 402 may take measurements at 200 logarithmic steps along thefrequency range. The frequency range for which measurements are to betaken may vary depending on the specifics of the embodiment but istypically over many orders of magnitude (e.g. 50-20 kHz, 10 mHz to 1MHz, etc.)

In another embodiment the current is applied at the selected frequencyand the voltage is measured. In this case block 508 in FIG. 5 would read“Apply current at selected frequency” and block 510 in FIG. 5 would read“Measure voltage”.

In another embodiment the applied voltage signal is not of a singlefrequency but comprises the sum of a plurality of frequencies. Since theresponse properties of tissue for small voltage perturbations can beconsidered linear the recorded current will then contain signals at thesame frequencies as those in the stimulating voltage waveform. Thesefrequencies can be separated with appropriate filtering to yield thesame information as if individual frequencies had been applied andmeasured sequentially as illustrated in FIG. 5. This method has theadvantage that it may be quicker to perform than the method described inFIG. 5 since all frequencies are applied simultaneously. It has thedisadvantage that it may be less accurate than the method described inFIG. 5 due to the additional filtering step.

Blocks 506-512 illustrate a simplified method of applying a frequencysweep and performing measurements for a selected electrode pair. Itshould, however, be understood that other mechanisms for applying afrequency sweep and obtaining measurements may be used. Further, thevoltages, number of measurements and frequency range of the sweep areexemplary only, and in other embodiments different values may be used.

At block 506, control circuit 402 selects the starting frequency (e.g.,50 Hz) and voltage for the sweep (e.g., 50 mV). At block 508, controlcircuit 402 directs signal generator 404 to begin the frequency sweep.In response, signal generator 404 applies a signal to the electrodes atthe specified frequency and voltage. The voltage selected at block 506is preferably fairly small (e.g. 50 mV) so that the Voltage/Current(V/I) characteristic for the medium to be measured can be consideredlinear over the voltage range of the applied sinusoidal signal. Further,in an embodiment, the selected voltage may be a sub-threshold voltage,so that a hearing percept is not caused by the applied signal and therecipient may be unaware that the measurements are taking place. Thesignal applied by signal generator 404 to electrodes 448 and 450 may bea fixed sinusoidal signal at the specified frequency and voltage.

As noted above, resistor 406 is in series with signal generator 404,electrode 448, the recipient's tissue 452, and electrode 450. Thus, thecurrent through resistor 406 corresponds to the current passing throughelectrodes 448 and 450. Control circuit 402 measures the current throughthe electrodes 448 and 450 at block 510. This measurement may includeboth the amplitude and phase of the current. In the illustratedembodiment, voltage measurement circuit 408 measures the voltage dropacross resistor 406 and provides the measured voltage to control circuit402, which converts the measured voltage to current using the formula:I=V/R, where I is the measured current, V is the measured voltage, and Ris the resistance of resistor 406. Further, the measured currents (orvoltages) can be converted to a total impedance for the electrodes 448and 450 and tissue 452 using the following formula:V_(tot)=V_(R)+I*R_(tot), where V_(tot) is the voltage supplied by signalgenerator 404, V_(R) is the voltage drop across resistor 406, I is themeasured current through resistor 406, and R_(tot) is the totalimpedance of electrodes 448 and 450 and tissue 452. This formula can berewritten as R_(tot)=(V_(tot)−V_(r))/I. Further, the measured voltages,currents, and impedances may be complex comprising real and imaginaryparts based on the measured amplitudes and phases.

Control circuit 402 determines if the frequency sweep is completed ornot at decision 512. If not, control circuit 402 increases the frequencyof signal generator 404 at block 514. As noted above, in an embodiment,the frequency sweep may range from 50 Hz to 20 kHz, with the controlcircuit taking 200 measurements logarithmically spaced between 50 Hz and20 kHz. Thus, in an embodiment, control circuit 402 may direct thesignal generator 404 to apply a signal at the next frequency (e.g., 51.5Hz, 53.1 Hz, . . . 19409.8 Hz, 20 kHz) for which the control circuit 402is to obtain a measurement.

Once the frequency sweep is completed and the measurements obtained, thecontrol circuit 402 determines, at decision 516, if measurements are tobe obtained for other electrode pairs. For example, in an embodiment,control circuit 402 obtains measurements for each electrode (paired withthe extra-cochlear electrode) of the cochlear implant. However, inembodiments, control circuit 402 only obtains measurements for a subsetof the electrodes.

If measurements are to be obtained for other electrode pairs, theprocess returns to block 504 and measurements are obtained for the nextelectrode pair. Once measurements are obtained for each electrode pairto be measured, the process proceeds to block 518 and measurements areanalyzed. The particulars of this analysis may vary depending on theparticular information sought. Exemplary mechanisms for analyzing andusing this data (e.g., presenting location information to a surgeon) arediscussed below.

FIG. 6A illustrates a platinum electrode surrounded by a cellular medium(i.e. tissue). FIG. 6A is provided to illustrate how impedancespectroscopy can be used to determine the properties of the mediumsurrounding the electrode. FIG. 6B illustrates an equivalent circuitmodel 600 of the system illustrated in FIG. 6A. As shown in FIG. 6B theequivalent circuit model 600 includes Constant Phase Elements (CPEs) 652and 654 that are used to describe the circuit properties of biologicalinterfaces, which are typically rough or non-uniform at the microscopiclevel. A description of the CPE circuit model will be discussed belowwith reference to FIGS. 7A-7B.

As shown in FIG. 6A, current from the electrode 648 (e.g., an electrode148 (FIG. 1)) travels from the electrode's surface 650 through thetissue via two paths: an intra-cellular path where the current passesthrough cells 632 of the tissue and an extra-cellular current path wherethe current travels around the cells 632.

As noted the electrode's surface 650 is non-uniform at the microscopiclevel and can be modeled as a CPE 652. Similarly, the walls of the cells632 may be modeled as a CPE 654. The resistance of the intra-cellularcurrent path is modeled as resistive element, R_(int) 656. Theresistance of the extra-cellular path is modeled as resistive element,R_(ext) 658.

FIG. 7A provides a diagram for illustrating how the uneven surface of anelectrode at the microscopic level can be modeled as a CPE. As shown,the electrode surface 702 may have one or more microscopic pores 704 andthe surface can be modeled as an imperfect capacitor.

FIG. 7B provides a diagram of a circuit model for a CPE in accordancewith FIG. 7A. The circuit model uses conventional electrical componentsand comprises a plurality of arms, each with a resistor 712 and acapacitor 714. The resistance and capacitance of each arm increase by afactor of N (e.g., N=2) for each arm of the circuit. The circuit modelincludes enough arms so that the RC time constants for the circuit spanthe frequency range being modeled. The CPE can be viewed as a circuitelement whose phase angle (angular difference between the phase of thevoltage sinusoid and the current sinusoid) remains the same, regardlessof the frequency applied to it. In an embodiment, the phase angle issuch that the current leads the voltage by around 45 degrees.

As noted above with reference to FIGS. 4-5, the data recorded for eachelectrode pair is in the form of the amplitude and phase of a sinusoidresulting from the applied signal. This data can also be represented asreal and imaginary parts of the complex impedance of the electrodes 448and 450 and tissue 452 (FIG. 4). This data can be plotted in a number ofways, each highlighting different features of the medium being tested.For example, the data can be plotted with the magnitude of the impedanceon the y-axis against frequency on the x-axis.

FIG. 8 is an exemplary plot for an electrode surrounded by a cellularmedium, such as shown in FIG. 6A. FIG. 8 provides a curve 800 of themeasured impedance amplitudes versus frequency for one of the measuredelectrodes 458 of the stimulating lead assembly. In the model of FIGS.6A-6B, at extremely high frequencies the CPEs 652 and 654 all have verylow impedance so the total impedance of the circuit becomes the parallelcombination of R_(int) and R_(ext). Hence, above a certain frequency,the system looks completely resistive. At very low frequencies the CPEs652 and 654 (representing the platinum/tissue interfaces) dominate thesystem and the magnitude of the interface CPEs 652 and 654 can bedetermined. The bumps in the curve 800 at intermediate frequencies arecaused by the presence of the cells and cell walls in the medium.Different cell types cause different inflections and plateaus in thisintermediate region of the plot. Thus, obtaining impedance measurementsfor plurality of frequencies, as shown, helps provide a mechanism fordistinguishing between impedances due to the electrode and those due tothe tissue.

In embodiments, the selected set of frequencies to be measured can betailored to the particular application. For example, the typical timetaken to run a full impedance sweep can be several minutes or hourssince typically the frequencies in the milli-Hertz region require longdata collection times. Since these low frequency measurements provideinformation mostly about the electrode/tissue interface and not itssurrounding structures, in certain embodiments (e.g., when used duringcochlear implant implantation), the method can omit many of the lowfrequency measurements. For example, when used during cochlearimplantation, the method may only take measurements for a subset of keyfrequencies that can be quickly obtained and provide sufficientinformation for identifying the location of the stimulating leadassembly.

In the above discussed embodiment of FIG. 4, a dedicated signalgenerator 404 (FIG. 4) was used for applying the signal to the electrodepair. This dedicated signal generator circuitry may be useful in forcinga sinusoidal voltage or current and measuring a resulting sinusoidalcurrent or voltage, respectively.

In other embodiments, the stimulating signal generation circuitry usedfor applying stimulation to a recipient in an implantable stimulatorsystem may be used to generate the signal(s) used to measure impedance.For example, cochlear implants typically include circuitry designed todeliver a square wave current pulse as part of their neural stimulatorfunction. Existing cochlear implants also typically include circuitryfor measuring a voltage drop across the electrodes (e.g., the voltagedrop across a particular electrode and the extra-cochlear electrode orthe voltage drop across two electrodes of the stimulating leadassembly). Embodiments of the present invention use the square wavecurrent pulse of the signal generation circuitry to obtain impedancespectroscopy data by measuring and storing the voltage across theelectrodes (e.g., the electrode of the stimulating lead assembly thatapplied the pulse and the extra-cochlear electrode) at various timesduring the pulse.

FIG. 9 illustrates a square wave current pulse 902 and a resultingmeasured voltage 904, in accordance with an embodiment of the presentinvention. In this embodiment, the stimulating signal generationcircuitry of the cochlear implant is used to apply the current pulse.Further, circuitry included in the stimulator unit is used to measurethe voltage across the electrode pair. Although data obtained in thismanner may not be exactly translated into the data obtained throughconventional impedance spectroscopy, it is closely related to it and islikely to provide useful information for deriving information about thestructures near the electrodes.

In an embodiment, the time during the pulse at which the voltage acrossthe electrodes is measured is closely related to the inverse of thefrequency in a conventional impedance spectroscopy system. For example,the voltage measured 10 μs after the start of the current pulse isclosely related to the amplitude data that would be obtained at afrequency of 100 kHz (= 1/10 μs) with a conventional impedancespectroscopy system. Similarly, voltage measured 100 μs after the startof a pulse is closely related to an impedance spectroscopy amplitudemeasured at a frequency of 10 kHz= 1/100 μs.

The Laplace transform can be used to relate the measured voltage duringthe stimulation pulse to the frequency spectrogram as follows: During(phase 1 of) a constant current stimulation pulse, assume the timedomain current is I(t) and the time domain voltage is V(t). I(t) andV(t) can be transformed using the Laplace operator to yield thefrequency domain current, I(s), and frequency domain voltage, V(s) asfollows.

For a constant current stimulator I(t)=I (a constant) so the frequencydomain current, I(s) is the Laplace transform of the constant I so:I(s)=I/s. The signal V(t) is measured numerically by the AIMD. TheLaplace transform, V(s), of the time domain voltage waveform, V(t), canbe calculated numerically from the measured values of time domainvoltage, V(t). i.e.: V(s)=L[V(t)] where L is the Laplace operator.

From the frequency domain current, I(s), and the frequency domainvoltage, V(s), the frequency domain impedance, also known as theimpedance spectrogram, can be calculated as: Z(s)=V(s)/I(s). As notedabove, the measured impedance data may be analyzed in various mannersdepending on the particular implementation. For example, in anembodiment, the raw current measurements may be provided by controlcircuit 402 (FIG. 4) to an external device that analyzes themeasurements to display information to a surgeon regarding the locationof the stimulating lead assembly. This external device may be, forexample, a computer or specialized piece of hardware and/or software.Or, for example, in an embodiment, the data is analyzed by cochlearimplant 100, such as by the sound processor 126 or control unit 402. Thecochlear implant 100 may analyze the data to, for example, determine ifa fault occurred or the stimulating lead assembly has shifted positionfrom its initial position. The cochlear implant (e.g., sound processor126) may then take appropriate action such as, for example, modifyingthe MAP used in generating electrical stimulation, providing anindication to the external speech processor that can be used to notifythe recipient or surgeon of an issue with the cochlear implant, or, forexample, terminate the application of stimulation to the recipient.

As noted, in an embodiment, control circuit 402 provides the raw data toan external device, such as a computer or specialized piece of hardwarethat analyzes the data. In such an embodiment, the external device maybe connected to the sound processor 126 (FIG. 1) by a wired or wirelessconnection. For example, in an embodiment, control circuit 402 providesthe data to internal receiver unit 132 (FIG. 1), which transmits thedata to external transmitter unit 128. External transmitter unit 128then provides the data to sound processor 126, which then provides thedata to the external device.

The external device may compute a curve, such as curve 800, from the rawdata and compare the computed curve to known curve patterns representingdifferent characteristics to identify the proximity of the electrode toa particular structure. Various techniques may be employed forperforming this comparison, such as, for example, by using a neuralnetwork to compare the measured curve to known curve patterns todetermine the proximity of the electrode to different structures. Forexample, the curve shape for an electrode close to the modiolus will bedifferent than the curve shape for an electrode that is located awayfrom the modiolus and surrounded by perilymph.

The following provides an exemplary description of an embodiment inwhich the measured information is provided to an external device so thatinformation on the location of the stimulating lead assembly may bepresented to a surgeon during implantation of the stimulation leadassembly. In an embodiment, this information is provided in real time sothat a surgeon implanting a stimulating lead assembly can obtain realtime data about the cochlear structures near the stimulating leadassembly. This information may be displayed to the surgeon in a visualform (for example a head up display of one or more of the impedancespectroscopy plots). Or, for example, this visual data may be presentedthrough the operating microscope as a head-up display. Or, for example,the data may be presented aurally as a single frequency continuous tone(e.g. the tone frequency or amplitude could be related to the impedancespectroscopy amplitude data measured at a particular frequency). Or thesound could be a continuous, complex sound (e.g. consisting of asummation of multiple tones, each tone related to the impedancespectroscopy amplitude at a range of frequencies). Or the sound could bea series of tone bursts where the tone burst frequency is related insome way to the salient impedance spectroscopy data.

As noted, in embodiments the data is presented in real time (so the toneor visual data changes more or less instantaneously as the stimulatinglead assembly is moved) or it may be presented after the fact. Theadvantage of real time is that it allows the surgeon to respondinstantly to any detected changes or problems associated with theproximity to neural structures. The advantages of “after the fact”presentation is that it allows more time for any post processing of datathat may be necessary in order to extract the most useful data topresent to the surgeon.

The data processing, salient feature extraction and presentation meanswill vary depending on the application required. For example, if asurgeon wishes to know the proximity of all the electrodes of a cochlearimplant stimulating lead assembly to the lateral wall of the cochlea,then, in an embodiment, data from all the electrodes of the array aremeasured, processed and presented to he surgeon, in more or less realtime.

In another embodiment, a clinician may wish to determine,postoperatively, the proximity of the electrodes of a stimulating leadassembly to the modiolus. This may provide useful data that affects theway the cochlear implant is programmed (e.g. there is evidence to showthat close modiolar positioning of the electrodes reduces current spreadand potentially allows the use of programming strategies requiringgreater spatial selectivity). In this case there is no need for realtime processing of the data since the clinician has ample to time toconsider the best way to program the cochlear implant. In this case itmay be advantageous to do more complex processing and presentation ofthe impedance spectroscopy data, perhaps presenting the data in a rangeof presentation styles and methods, since additional informationrelevant to modiolar proximity may be obtained through greaterprocessing of the raw data.

In addition to plotting the measured impedance amplitudes versusfrequency and comparing the plot to known patterns, embodiments of thepresent invention may also analyze the data in different ways dependingon the particular physical characteristics of the cells or electrodethat are to be determined. These other mechanism include, for example,plotting (a) the phase angle of the complex impedance vs. frequency; (b)the real component of the complex impedance vs. frequency; (c) theimaginary component of the complex impedance vs. frequency; and (d) thereal (e.g., x-axis) vs. imaginary (y-axis) impedance components for eachfrequency measured. Further, in another embodiment, rather than simplyusing the raw data, the data may be further processed. For example, inan embodiment, the derivative of the measured impedance (or voltage orcurrent) vs. time is obtained and the resulting derivative is used, forexample, to obtain information regarding the stimulating lead assembly,such as information regarding the proximity of the electrode to thestructure of the medium surrounding the tissue. In one such embodiment,the resulting derivative values are plotted versus frequency anddisplayed to the surgeon, or for example, used as inputs to aclassification algorithm (e.g., neural network) such as discussed above.

In an embodiment, the impedance spectroscopy data may be combined (e.g.,by an external device) with other data relating to structures near theelectrode (e.g., x-rays and other imaging data, optical measurements,force measurements, etc.).

It should be noted that the above description provides one example forobtaining impedance spectroscopy measurements, and in other embodiments,more complex impedance spectroscopy measurements may be obtained. Forexample, in the embodiment the current is measured on the same electrodeas the one that applies the voltage. It is also possible to applycurrent or voltage on one electrode and measure voltage or currentrespectively on one or more other electrodes in the array of electrodes.Applying voltage or current on one electrode and measuring the voltageor current on another electrode is known to those of skill in the artand is not discussed further herein. The system of forcing current onone electrode and then measuring voltage on other electrodes in an arrayof electrodes is sometimes referred to as electrode field imaging.

As noted above, impedance spectroscopy data may be used for determiningproximity information. Additionally, as noted above, the impedancespectroscopy data may be used for obtaining other types of information.For example, the impedance spectroscopy data may be used to determine iftip fold over occurs during surgical implantation of a stimulating leadassembly. When tip fold-over occurs, the electrode spacing is changed,such that one or more electrodes may be located very near anotherelectrode. In embodiments in which impedance spectroscopy data is usedto detect tip fold over, a current or voltage may be applied on one ormore electrodes and measured on one or more different electrodes. If tipfold over occurs the current or voltage applied on the electrode(s) maybe received on the other electrode(s) with a resulting distinctiveprofile that may be used to detect tip fold over. In such an embodiment,a classification algorithm (e.g., a neural network) may be used todetermine if the measured values indicate that tip fold over hasoccurred.

In another embodiment, the impedance spectroscopy data is used to detectwhether there is a fault or whether the tissue includes diseased ordamaged cells. In such an embodiment, these faults, as well as issueswith the tissue may each have distinctive impedance spectroscopycharacteristics. In an embodiment, a classification algorithm (e.g.,neural network) may be used to analyze the measured impedancespectroscopy data to determine if any of these issues has occurred.

In another embodiment, impedance spectroscopy measurements are obtainedusing a four point impedance method. In this method the voltage betweentwo nearby electrodes is measured while passing current between twoother electrodes flanking the measurement electrodes. A furtherdescription of an exemplary four point impedance method is provided inU.S. Patent Publication No. 2011/0087085 entitled “Method and Device forIntracochlea Impedance Measurements.”

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, operation, or other characteristicdescribed in connection with the embodiment may be included in at leastone implementation of the invention. However, the appearance of thephrase “in one embodiment” or “in an embodiment” in various places inthe specification does not necessarily refer to the same embodiment. Itis further envisioned that a skilled person could use any or all of theabove embodiments in any compatible combination or permutation.

It is to be understood that the detailed description and specificexamples, while indicating embodiments of the present invention, aregiven by way of illustration and not limitation. Many changes andmodifications within the scope of the present invention may be madewithout departing from the spirit thereof, and the invention includesall such modifications.

What is claimed is:
 1. A method of operating an active implantablemedical device (AIMD) that is a cochlear implant comprising at least oneelectrode, the cochlear implant being configured to execute an impedancespectroscopy method, the method comprising: applying a measurementsignal at a plurality of frequencies to a recipient of the AIMD usingthe electrode; performing, using the AIMD, a measurement, responsive tothe measurement signal and indicative of an impedance of the electrodeand tissue of the recipient, at each of the plurality of frequencies;analyzing the measurements to obtain information regarding the proximityof the electrode to one or more tissue structures inside a cochlea; andat least one of: assessing an extent of scar tissue around the electrodebased on the measurements; determining a presence of an infectionproximate the electrode based on the measurements; or determining, basedon the measurements, a complication associated with the electrode. 2.The method of claim 1, further comprising: analyzing the measurements todetermine location information regarding the electrode.
 3. The method ofclaim 1, wherein the analyzing comprises: analyzing the measurements todetermine information regarding a proximity of the electrode to one ormore tissue structures.
 4. The method of claim 3, wherein the one ormore tissue structures comprise a modiolus.
 5. The method of claim 1,wherein the analyzing is performed during implantation of a component ofthe AIMD, wherein the component comprises the electrode.
 6. The methodof claim 5, further comprising displaying on a device external to theAIMD the location information during the implantation.
 7. The method ofclaim 6, where the displaying comprises: displaying the information on aheads-up display.
 8. The method of claim 1, wherein: the action ofapplying a measurement signal includes applying a first measurementsignal at a first of the plurality of frequencies with a specifiedelectrical characteristic using the electrode; the method furthercomprises: applying a second measurement signal at a second of theplurality of frequencies with the specified electrical characteristicusing the electrode, wherein the action of performing a measurementincludes: measuring a first impedance of the electrode and tissue inresponse to the first measurement signal at the first of the pluralityof frequencies; and measuring a second impedance of the electrode andtissue in response to the second measurement signal at the second of theplurality of frequencies.
 9. The method of claim 8, further comprising:computing a curve using the first and second impedances; and comparingthe computed curve to one or more known curve patterns to identify theproximity of the electrode to one or more tissue structures.
 10. Themethod of claim 9, wherein comparing comprises: using a neural networkin comparing the computed curve to one or more known curve patterns. 11.The method of claim 1, wherein the measurement signal is applied by asignal generator of the AIMD implanted in the recipient that isconfigurable to apply at least one stimulation signal using theelectrode to cause a hearing percept by the recipient.
 12. The method ofclaim 1, wherein the cochlear implant comprises an extra-cochlearelectrode and a stimulating lead assembly comprising the electrode and aplurality of other electrodes, wherein the applying comprises: applyingthe measurement signal using the electrode and the extra-cochlearelectrode; and wherein the method further comprises: applying a secondmeasurement signal at a plurality of frequencies to the recipient usingone of the plurality of other electrodes and the extra-cochleaelectrode; and performing, using the AIMD, a second measurementresponsive to the second measurement signal and indicative of animpedance of the one of the plurality of other electrodes and tissue ofthe recipient, at each of the plurality of frequencies.
 13. The methodof claim 1, where the applying and performing are performed for each ofthe plurality of frequencies using a frequency sweep.
 14. The method ofclaim 13, wherein the plurality of frequencies of the frequency sweepcomprise frequencies logarithmically spaced across a frequency range ofthe frequency sweep.
 15. The method of claim 1, further comprising:adjusting one or more operating parameters of the AIMD based on themeasurements.
 16. The method of claim 1, further comprising: emitting anaudible signal based on the analysis.
 17. The method of claim 1,wherein: the AIMD includes a plurality of separate stimulation channels;and the method includes performing at least one of impedancemeasurements or voltage measurements for the plurality of channels usingrespective different electrode pairs corresponding to respectivechannels, and for each respective pair, at least one of impedancemeasurements or voltage measurements are taken for a plurality offrequencies.
 18. The method of claim 1, further comprising: evaluatingthe measurements to differentiate between different physical structuresin the recipient that have different spectroscopic signatures, whereinthe AIMD is implanted in a head of a recipient with the electrode in thecochlea.
 19. The method of claim 1, wherein: the AIMD includes astimulating lead assembly; and the method further comprises determiningan instantaneous insertion depth of the stimulating lead assembly in acochlea the cochlea of the recipient.
 20. The method of claim 1, furthercomprising at least one of: assessing the extent of scar tissue aroundthe electrode based on the measurements; or determining the presence ofan infection proximate the electrode based on the measurements.
 21. Themethod of claim 1, wherein: the AIMD includes an electrode array havinga tip inserted into the cochlea of the recipient; and the method furthercomprises determining whether the tip of the electrode array is in afold-over state based on a distinctive data related to the measurementsindicative of such a fold-over state.
 22. The method of claim 21,further comprising: utilizing a classification algorithm to determinewhether the tip of the electrode array is in a fold-over state.
 23. Themethod of claim 1, wherein: the applying action and the performingaction are executed during surgical implantation of the AIMD.
 24. Themethod of claim 1, further comprising assessing the extent of scartissue around the electrode based on the measurements, wherein theelectrode is located in the cochlea of the recipient.
 25. The method ofclaim 1, further comprising: distinguishing between impedances due tothe electrode and impedances due to the tissue.
 26. The method of claim1, further comprising: determining a magnitude of an impedance at aninterface between the electrode and the tissue.
 27. The method of claim1, further comprising: determining a first magnitude of a firstimpedance at a first interface between the electrode and the tissue; anddetermining a second magnitude of a second impedance at a secondinterface between another electrode different than the electrode of thefirst interface and the tissue.
 28. The method of claim 1, furthercomprising: evaluating at least one of a position of the electrode or atissue quality of the tissue based on the measurements by taking intoaccount the influence of the impedance of the electrode as compared tothe impedance of the tissue on the measurements.
 29. The method of claim1, further comprising: analyzing the measurements and predictingperformance of the AIMD based on the measurements.
 30. The method ofclaim 1, further comprising: surgically implanting a cochlear electrodearray into the cochlea based on the measurements.
 31. The method ofclaim 13, wherein: at least one of: the frequency sweep includes takinga plurality of steps along the frequency range; the frequency sweepincludes taking a plurality of logarithmic steps along the frequencyrange; or the frequency sweep includes skipping at least 199 frequenciesbetween 50 Hz and 20 kHz; and making one or more adjustments associatedwith the AIMD based on the measurements.
 32. The method of claim 1,wherein: the method further comprises displaying on a device external tothe AIMD information about the measurements and adjusting one or moreoperating parameters of the AIMD based on the measurements; and theaction of displaying on the device external to the AIMD informationabout the measurements to a person is executed through an operatingmicroscope.
 33. The method of claim 29, wherein: the action of applyinga measurement signal at a plurality of frequencies to a recipient of theAIMD using the electrode includes taking a plurality of steps along thefrequency range.
 34. The method of claim 1, further comprising:analyzing the measurement to determine a fault regarding the AIMD,wherein: the fault is a fault that is more complex than simple opencircuits or short circuits.
 35. The method of claim 1, wherein: anaction of displaying on the device external to the AIMD informationabout the measurements to a person is displayed to a surgeon implantingthe AIMD in real time with respect to the implantation.
 36. The methodof claim 1, wherein: the method further comprises making one or moreadjustments associated with the AIMD based on the measurements; and theaction of applying a measurement signal at a plurality of frequencies toa recipient of the AIMD using the electrode includes skipping at least199 frequencies between 50 Hz and 20 kHz.
 37. The method of claim 1,wherein: the method further comprises making one or more adjustmentsassociated with the AIMD based on the measurements; and the action ofapplying a measurement signal at a plurality of frequencies to arecipient of the AIMD using the electrode includes taking a plurality oflogarithmic steps along an operational frequency range of the AIMD. 38.The method of claim 1, wherein: the method further comprises making oneor more adjustments associated with the AIMD based on the measurements;and the action of applying a measurement signal at each of the pluralityof frequencies is executed without causing a hearing percept.
 39. Themethod of claim 1, wherein: the method further comprises making one ormore adjustments associated with the AIMD based on the measurements; andthe action of applying a measurement signal at each of the plurality offrequencies is executed during a surgical implantation operation of theAIMD.
 40. The method of claim 1, wherein the cochlear implant comprisesan extra-cochlear electrode and a stimulating lead assembly comprisingthe electrode and a plurality of other electrodes located in thecochlea, and wherein the measurement signal does not cause a hearingpercept in the recipient.
 41. The method of claim 1, wherein the tissueis tissue of the cochlea.
 42. The method of claim 1, wherein thecochlear implant comprises an extra-cochlear electrode and a stimulatinglead assembly comprising the electrode and a plurality of otherelectrodes located in the cochlea, and wherein the measurement signalcauses a hearing percept in the recipient.
 43. The method of claim 1,wherein the analyzing is performed during implantation of a component ofthe AIMD, wherein the component comprises the electrode, and wherein themethod further comprises providing data to a surgeon indicative of astate of the AIMD during implantation thereof, which data is based onthe measurements.
 44. The method of claim 29, wherein: the measurementsis indicative of proximity of the electrode to a modiolus, and whereinthe AIMD is the cochlear implant.
 45. The method of claim 28, wherein:the action of evaluating at least one of a position of the electrode ora tissue quality of the tissue based on the measurements by taking intoaccount the influence of the impedance of the electrode as compared tothe impedance of the tissue on the measurements includes evaluating theposition of the electrode based on the measurements by taking intoaccount the influence of the impedance of the electrode as compared tothe impedance of the tissue on the measurements.
 46. The method of claim38, wherein the electrode is proximate tissue structure, wherein thetissue structure comprises tissue of the cochlea.
 47. The method ofclaim 1, further comprising determining the presence of an infectionproximate the electrode based on the measurements.
 48. The method ofclaim 1, further comprising determining, based on the measurements, thecomplication associated with the electrode.